Acoustic pyrometer

ABSTRACT

An acoustic pyrometer measures the average gas temperature across a wide space of known distance, especially turbulent, high temperature gas loaded with caustic particulates. It includes an acoustic signal generator that generates a high amplitude acoustic signal with a short rise time and a detector positioned adjacent the signal generator that detects the onset of the acoustic signal in the signal generator and generates a first electrical signal corresponding in time to the onset of the acoustic signal in the signal generator. A receiver, positioned across the space from the signal generator, receives acoustic signals from the space and generates electrical signals corresponding to amplitude and frequency of the acoustic signals received in the receiver. A signal processor processes the electrical signals from the receiver to distinguish the onset of the acoustic signal from background noise in the space as detected in the receiver, and processes the electrical signals from the receiver to produce a distinct differentiation between background noise and the onset of the acoustic signal in the receiver. The signal processor then compares the time of the onset of the acoustic signal in the receiver with the onset of the acoustic signal in the signal generator to determine the transit time of the acoustic signal to traverse the space, and also calculates the temperature of the gas in the space based on the transit time.

This application is a Divisional of application Ser. No. 10/020,106filed Dec. 14, 2001, which is a Continuation of Ser. No. 09/462,325, nowU.S. Pat. No. 6,386,755, filed on Jan. 5, 2000, which is a 371 ofPCT/US98/13839 filed on Jul. 4, 1998, which claims benefit of 60/052,930filed on Jul. 5, 1997.

This invention pertains to temperature measurement, and moreparticularly to measurement of temperatures across large spaces of knowndistance in a noisy, dirty and corrosive environment such as acoal-fired utility boiler, or a chemical recovery boiler.

BACKGROUND OF THE INVENTION

Coal-fired boiler operations are significantly influenced by operationalparameters that vary with changing environmental factors, includingambient temperature, humidity, coal composition, etc. Gas temperaturesin the boiler, including furnace exit gas temperatures, are influencedby these factors, as well as by adjustments that can be made to thefurnace apparatus, such as burner configuration and orientation, airflow rate, coal feed rate, etc.

Gas temperatures profoundly affect the performance of a boiler inseveral ways. The thermal NO_(X) formation rate increases exponentiallyat temperatures over 2700° F. There is strong regulatory pressure toreduce NO_(X) emissions, but the fundamental knowledge of furnace exitgas temperatures, the primary factor in NO_(X) formation, is lacking inlarge boilers because existing temperature measurement technology isincapable of producing accurate temperature data in large boilers.

Boiler gas temperatures also influence slag formation rates on boilertubes. Slag is an accumulated deposit of materials present in coal thatare formed as ash particles when the coal is burned in the furnace butwhich impinge and stick on the pendant steam/water tubes when the gastemperature is near the fusion temperature of the ash particles (theso-called “sticky zone”). Slagging of the tubes is a common phenomenonin all coal fired boilers, but is particularly troublesome in thoseboilers using sub-bituminous coal such as the low sulfur coal from thehuge deposits in the Power River Basin. Slag is a problem because itinterferes with heat transfer to the boiler tubes, and can impede gasmovement through the tube banks. Even more serious, slag deposits cangrow to enormous size and then fall, causing extensive damage to theboiler and resulting in expensive boiler down-time while repairs aremade.

Boiler tubes are cleaned of slag deposits by “soot blowing” blasts ofsteam injected through vents in rotating pipes, but the frequency andlocation of the soot blowing is based primarily on guesswork by theoperator rather than a real knowledge of the actual current conditionsin the boiler that produce slagging of the boiler tubes. Soot blowingreduces the efficiency of the boiler and can itself cause erosion of thetubes, so there is a strong incentive to optimize the soot blowingoperation, that is, to operate it only with the necessary frequency,duration and location. One technique to determine when the tubes arebecoming slagged is by measuring the temperature on opposite sides of abank of tubes to ascertain how much heat is being transferred throughthe tubes to the water/steam in the tubes. When the temperaturedifferential drops, that is an indication that the tubes are becomingslagged since the slag acts as an insulator, retarding the heattransfer. However, there must be an accurate measure of the gastemperatures on opposite sides of the tube banks for the temperaturedifferential scheme to work, and state of the art temperaturemeasurement techniques are inaccurate or short lived for large boilerinstallations.

A better approach to the slagging problem would be to minimize theformation of slag and thereby reduce the need to remove it. Since slagformation is influenced by gas temperature, a knowledge of thetemperatures at the inlet plane to the tube bank, and/or in the tubebank itself would enable the boiler operator to determine when thethermal conditions are approaching those under which tubes are likely tobecome slagged. Control of gas temperature to prevent the creation ofthe “sticky” zone of slag formation would help to delay the onset ofboiler pluggage and forced shutdown for cleaning. Detailed knowledge ofthe thermal conditions in the region of the tube bank can be helpful,not only in assessing where slagging is likely to occur, so that sootblowing may be optimized for the conditions, but also can be used inadjusting the furnace to produce gas temperatures which minimizeslagging.

Thus, there has long been a need for accurate temperature measurementsin large power and recovery boilers that enable improvements to be madein boiler efficiency, and also reduce the formation of slag and optimizesoot blowing to remove slag that does form so that large slag depositsdo not form and cause boiler damage from slag falls. The temperaturemeasurement would also be useful in minimizing NO_(X) formation byreducing the dwell time at high temperature. Finally, such a temperaturemeasurement would facilitate furnace fireball centering, firewall flameimpingement detection, and tube leak detection.

The long felt need for accurate temperature measurement in large boilersexists because the prior art measurement techniques are inadequate toreliably produce accurate temperature measurement across the width oflarge boilers. Thermocouple probes are unreliable and fail quickly incorrosive environments. Optical methods have limited penetration and aredifficult to interpret. Prior acoustic methods cannot operate accuratelyover large spans in noisy environments, in part because they are unableto accurately detect the onset of the acoustic signal in high amplitudebackground noise.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedacoustic pyrometer that can make accurate measurements of elevated gastemperatures across wide spaces in the presence of substantial acousticnoise. Another object of this invention is to provide an improvedacoustic pyrometer capable of accurate operation in atmosphericconditions wherein the path length before absorption of opticalwavelengths used in optical pyrometry are short and optical pyrometry isdifficult to interpret. It is another object of this invention toprovide an improved acoustic pyrometer having the capability ofperforming advanced diagnostic functions regarding the internaloperation of a boiler, e.g. to facilitate optimal furnace adjustment forfireball centering, firewall flame impingement detection. It is yetanother object of this invention to provide improved methods formeasurement of heat transfer to boiler tubes, tube leak detection,localization of slag-formation regions, furnace plane temperaturemapping, and optimizing soot blowing operations.

These and other objects of the invention are attained in an improvedacoustic pyrometer for measuring the average gas temperature across anopen space of a known distance. It includes an acoustic signal generatorfor generating an acoustic signal with a high amplitude sudden onset orshort rise time. A detector is positioned adjacent the signal generatorin a position to detect the onset of the acoustic signal in the signalgenerator. The detector could be an acoustic signal detector such as amicrophone or a piezo-electric detector, or it could be a proximity ortranslation detector that senses the movement of the signal generatorcomponent that releases the acoustic signal. The detector generates afirst electrical signal corresponding in time to the onset of theacoustic signal in the signal generator. A receiver is positioned acrossthe space of known distance from the signal generator, and has amicrophone or other acoustic signal sensor for receiving acousticsignals from the space and for generating electrical signalscorresponding to amplitude and frequency of the acoustic signalsreceived in the receiver. The signals from the acoustic signal detectorassociated with the signal generator and the acoustic signal sensor inthe receiver are sent to a signal processor, to distinguish the acousticsignal from background noise in the space as detected in the receiver,and for comparing the time of arrival of the acoustic signal in thereceiver with the time when the acoustic signal was generated in thesignal generator to determine the transit time of the acoustic signal totraverse the space, and for calculating the temperature of the gas inthe space based on the transit time.

DESCRIPTION OF THE DRAWINGS

The invention and its many attendant objects and advantages will becomebetter understood upon reading the following detailed description of thepreferred embodiments in conjunction with the following drawings,wherein:

FIG. 1 is a schematic perspective view of a coal-fired boiler with anacoustic pyrometer system in accordance with this invention;

FIG. 2 is a perspective view of a signal generator of the acousticpyrometer of this invention shown in FIG. 1, mounted on a tube wall of aboiler;

FIG. 3 is a sectional plan view of a boiler tube wall with a mountingcoupling by which the signal generator is mounted;

FIGS. 4 and 5 are front and side elevations of a coupling mounted on thetube wall shown in FIG. 3;

FIG. 6 is a schematic view of a fire box in a tangentially fired boiled;

FIGS. 7 and 9 are schematic views of a boiler with an acoustic pyrometerof this invention mounted thereon;

FIG. 8 is simplified schematic view of the air system for powering theacoustic pyrometer of this invention;

FIGS. 10 and 11 are schematic pneumatic and electrical diagrams for anacoustic pyrometer in accordance with this invention;

FIG. 12 is a side elevation showing a mechanical structure for mountingthe signal generator shown in FIG. 10 on the tube wall of a boiler;

FIGS. 13-17 are assorted views of modified view port elements used formounting the signal generator shown in FIG. 12 to a view port of aboiler in accordance with this invention

FIG. 18 is a sectional schematic elevation of the signal generator shownin FIG. 12 in the ready-to-fire configuration;

FIG. 19 is a sectional schematic elevation of the signal generator shownin FIG. 18 with the valve opened and the seal between the front and rearchambers just broken;

FIG. 20 is a sectional schematic elevation of the signal generator shownin FIG. 18 with the piston accelerated rapidly to the rear, opening thefront seal and releasing the acoustic signal which is shown propagatingdown the barrel;

FIG. 21 is a sectional schematic elevation of the signal generator shownin FIG. 18 with the piston being decelerated by a gas cushion trapped inthe end of the rear cylinder;

FIG. 22 is a schematic perspective view of the interior of a boilershowing an array of signal generators and receivers arranged in thefurnace exit plane in a position to produce a thermal map of thetemperatures in the furnace exit plane;

FIGS. 23 and 24 are schematic plan views of a furnace with parallelopposed burners and an acoustic pyrometer in accordance with thisinvention arranged to produce firewall impingement temperature data;

FIG. 25 is a schematic side elevation of a boiler with an acousticpyrometer in accordance with this invention arranged to produce dataregarding the vertical temperature gradient for management of thermalNo_(X) in the boiler;

FIG. 26 is a sectional elevation of a signal generator for a acousticpyrometer in accordance with this invention;

FIG. 26A is an end elevation of one of the hangers shown in FIG. 26;

FIG. 27 is a sectional elevation of the signal generator shown in FIG.26 with the shaft/piston removed;

FIG. 28 is a sectional elevation of the integral piston/shaft shown inFIG. 26, with all components removed for clarity of illustration;

FIG. 28A is an exploded sectional elevation of the plug end of theintegral piston/shaft shown in FIG. 28;

FIG. 29 is a sectional elevation of the signal generator shown in FIG.26 with the seal between the front and rear chamber just broken;

FIG. 30 is a sectional elevation of the signal generator shown in FIG.26 with the front seal opened and releasing the acoustic signal;

FIGS. 31-33 are block diagrams showing the operation of the signalprocessor for an acoustic pyrometer in accordance with this invention;and

FIGS. 34-38 are schematic views of boilers showing several specializedfunctions that can be performed by an acoustic pyrometer in accordancewith this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, wherein like reference numerals designateidentical or corresponding parts, and more particularly to FIGS. 1-3thereof, a coal fired boiler 30 is shown schematically having walls 32formed of steam/water tubes 35 joined edge-to-edge with webs 36, asshown in FIG. 3. Pendent steam/water tube loops 38 extend into a streamof hot combustion gas products from a furnace 40 which burns powderedcoal. The furnace 40 may have various burner configurations, such as aneffective arrangement shown in FIG. 6 having a series of adjustableburners 42 arranged tangentially around a central area to produce aswirling fireball 44 centered in the furnace.

The pendant tubes 38 occupy an open space within the boiler that canexceed fifty feet across. For example, a B&W 670 Megawatt boiler has aspan of about 67 feet. The atmosphere inside an operating coal-firedboiler is typically dirty, with a high concentration of suspendedparticulates, and is also very noisy because of the turbulence of thecombustion gases, boiling of water in the tubes 35 and 38, and operationof soot blowers (cleaning devices) inside the boiler to prevent slagbuild-up on the boiler. These conditions present an extremely hostileenvironment for operation of gas temperature measurement systems withinthe boiler.

An acoustic pyrometer 50, shown schematically in FIGS. 7-11, measuresthe average gas temperature along one or more paths 52, 52A, 52B, etc.across the open space in the boiler or other open space of a knowndistance. The acoustic pyrometer 50 includes an acoustic signalgenerator (ASG) 55 for generating an acoustic signal 56 with a fast risetime and high amplitude. A means of detection such as a microphone 57,shown in FIGS. 10 and 11, is connected to the acoustic signal generator55 through a tube 59 in a position to detect the generation of theacoustic signal produced by the acoustic signal generator 55. Othertypes of detection means are contemplated also, as described below. Themicrophone 57 detects the onset of the signal 56 in the acoustic signalgenerator 55 and generates a first electrical signal correspondingclosely in time to the generation of the acoustic signal 56 in theacoustic signal generator 55.

An acoustic signal receiver (ASR) 60 is positioned across the boiler orother space of known distance from the signal generator 55, and has amicrophone 62 or other transducer for receiving acoustic signals fromthe space and for generating electrical signals corresponding toamplitude and frequency of the acoustic signals received in the receiver60. The electrical signals from the signal detector 57 and the receivermicrophone 62 are sent via electrical conductors 63 and 64 to a signalprocessor 65 in a system controller 66 for processing, to distinguishthe arrival of the acoustic signal generated by the acoustic signalgenerator 55 from background noise in the space as detected in thereceiver 60, and for comparing the time of the arrival of the acousticsignal in the receiver 60 with the time of generation or onset of theacoustic signal in the signal generator 65 to determine the transit timeof the acoustic signal to traverse the space, and for calculating thetemperature of the gas in the space, based on the transit time.

The microphone 57 is conveniently mounted in an enclosure for anacoustic signal trigger 68 which provides the electrical interfacebetween the signal processor 65 and the acoustic signal generator 55.Alternatively, the means for detecting the onset of the signal in thesignal generator 55 could be sensors in the signal generator itself.Such sensors could be acoustic sensors such as a microphone orpiezoelectric devices, or could be motion or proximity sensors of theelectrical or optical varieties that sense movement of components in thesignal generator that move to release pressurized gas that creates theacoustic signal 56. Other sensors are possible as will be appreciated bythose skilled in the art.

Typically, as illustrated in FIG. 1, the acoustic signal generator 55 ismounted in one tube wall 32 of the boiler and the receiver 60, or asshown in FIG. 1, three receivers 60 are mounted in the opposite tubewall directly over the furnace 40 to give information about thetemperature distribution of the gas at the furnace exit plane. In FIG.2, the acoustic signal generator 55 is mounted in a side tube wall abovethe bull nose tube wall 69 and in front of the first row of pendenttubes 38, and three receivers 60 are mounted in the opposite side tubewall, also in front of the first row of pendant tubes 38 to obtaininformation about vertical distribution of the gas temperatures as thegas enters the pendant tube banks. One convenient and minimally invasivemounting technique for the receiver 60, shown in FIGS. 3-5, is to cut anarrow slot 75 through the web 36 connecting adjacent tubes 35 of thetube wall 32. A conventional coupling 77 is welded to the web above andbelow the slot and preferably the coupling is also supported by top andbottom gussets 79 welded in place. The receiver 60 can be mounted on thetube wall in this fashion without modifications of, or interferencewith, the operation of the tube wall 32 and without using any of theexisting ports. Another mounting technique uses the existing ports inthe tube wall, as shown in FIGS. 12-19. A port door 80 over the existingport is removed and an adapter 82 is secured in its place using the portdoor hinge knuckles and latch to secure the adapter 82 in place.

The signal generator 55 may be mounted to the adaptor 82 on the tubewall by conventional mounting brackets or the like, although a hangersystem 84 shown in FIG. 12 is preferred. The hanger system 84 includes ahanger beam 85 supported at one end on a connector 86 attached to thetube wall 32, and at the other end to diagonal brace 87 extending downfrom another coupling (not shown) at the tube wall 32 above the coupling86. The acoustic signal generator 55 has a pair of yokes 88, each havinga wheel 89 at its upper end by which the signal generator 55 issupported on the rail 85 to accommodate recoil movement when the signalgenerator operates. A pair of compression springs 90 on opposite sidesof one of the yokes absorbs the recoil energy and returns the signalgenerator to its start position after each operation.

The acoustic signal generator 55 generates an acoustic signal with ahigh amplitude, fast rise time. The acoustic signal generator, shown inFIGS. 20-23, includes a main cylinder 91 having front and rear opposedend closures 92 and 93, respectively, at front and rear axial openingsin the front and rear ends of the cylinder 91, respectively. An internalpartition 95 at an intermediate portion of the main cylinder divides thecylinder into front and rear chambers 96 and 97, respectively, and anaxial port 99 in the partition 95 communicates between the front andrear chambers 96 and 97. The rear end closure 93 has openings 94 toallow air to move freely into and out of the rear chamber 97 as shown inFIG. 22. A rear cylinder 100 is attached to the rear closure 93 around arear axial opening 102 which provides communication therethrough betweenthe rear chamber 97 and the rear cylinder 100. A piston assembly 105 hasan intermediate piston 107 in the rear chamber 97, and a rear piston 108in the rear cylinder 100. A seal assembly 110 is connected to the frontend of the piston assembly 105, specifically, to the front face of theintermediate piston 107, for movement therewith. The seal assembly 110has a front plug seal 112 plugging a front end axial opening 113 in thefront closure 92, and has an intermediate seal 114 plugging the axialport 99 in the partition 95. The intermediate seal 114 is preferablyslightly larger than the front seal 112 so there is an unbalancedpneumatic force exerted by gas pressure in the front chamber 96 on theseal assembly 110 tending to open the seal assembly 110 to the rear. Thefront plug seal 112 includes a plug 116 with a sliding seal 118 disposedin the front axial opening 113 and movable axially therein. The nozzleor barrel 70 of the signal generator is attached to the front closure 92of the main cylinder 85 in communication with the front opening 113.

A pneumatic operating system, shown schematically in FIGS. 10 and 20-23,is provided for charging the front chamber 96 of the main cylinder 91with gas at a first high pressure and for charging the rear cylinder 100with gas at a second high pressure. The gas could be air, carbondioxide, nitrogen, or some other suitable gas. The pneumatic operatingsystem includes a high pressure gas supply 125, shown in FIG. 10,coupled via an gas filter 126 in an inlet gas line 127 to a source ofgas pressure 129, such as a plant pressure supply line. A pressureamplifier 130 of conventional design raises the gas pressure supplied bythe source 129 to an elevated pressure on the order of 250-400 PSI,preferably 400 PSI, to be supplied through an inlet gas line 132 via a3-way solenoid valve 135 to the rear cylinder 100 of the signalgenerator 55. An extension 133 of the inlet gas line 132 suppliespressurized air to the front chamber 96 through a restriction 134. Therestriction 134 could be a reduction in the internal diameter of the gassupply line 133 to the front chamber 96 or could be an apertured orporous plug inserted in the line 133.

The three-way solenoid valve 135 is remotely operated by the controller66 to admit the pressurized gas from the high pressure gas supply 125through a restriction 137 into the rear cylinder 100, and, whenoperated, to close off the signal generator 55 from the high pressuregas system 125 and to allow gas to escape from the rear cylinder 100through an integral bleed vent in the valve 135 at a selected rate. Thepressure in the rear cylinder 100 drops faster than the pressure in thefront chamber 96 because the restriction 134 to the front chamber 96 ismuch smaller than the restriction 137 to the rear cylinder. When thepressure in the rear cylinder 100 has dropped sufficiently because ofthe escape of air through the restriction 137 and the bleed vent in thevalve 135, the sum of forces acting on the piston assembly 105 moves itto the rear as shown in FIG. 21. Specifically, the sum of the forwardlydirected forces on the piston assembly 105 exerted by pressurized gas onthe rear piston 108 in the rear chamber plus the forwardly directedforces exerted by the pressurized gas in the front chamber 96 on thefront plug seal 112 drops below the rearwardly directed forces exertedby pressurized gas in the front chamber 96 against the intermediate seal114. The piston assembly 105 moves to the rear, unsealing the axial port99 and allowing the full gas pressure in the front chamber 97 to actagainst the front face of the intermediate piston 107. The pistonassembly 105 and the attached seal assembly 110 is propelled at highacceleration to the rear under the influence of the unbalanced pneumaticpressure against the front face of the intermediate piston 107. As shownin FIG. 21, the front seal 118 is not yet opened while the pistonassembly is accelerating to the rear.

As shown in FIG. 22, the accelerating piston assembly 105 moves to therear far enough to pull the plug 116 clear of the front axial opening113 in the front closure 92 of the main cylinder 91. Because the pistonassembly 105 is moving fast by the time the seal 118 opens, the plug 116moves from a closed position to a fully opened position shown in FIG. 22in a very short time, typically less than 3 milliseconds. This allowsthe pressurized air in the front chamber 96 to escape explosivelythrough the nozzle 70, producing a high amplitude acoustic signal havinga sudden sharp onset, that is, a fast rise time.

The piston assembly 105 continues its rearward travel, driving thepiston 108 toward the rear end of the cylinder 100. The restriction 137vents pressure from the rear cylinder 100 through the bleed vent in thevalve 135 slowly enough that the piston 108 can compress the air in thecylinder 100 to function as a gas cushion, slowing and stopping therearward movement of the piston assembly 105 and preventing hard contactbetween the rear piston 108 with the rear end of the cylinder 100.

The valve 135 is now operated to connect the main cylinder 91 to thehigh pressure gas supply system 125. The gas enters the rear cylinder100 slowly, through the restriction 137, so that the intermediate piston107 makes soft contact with the partition 95. Simultaneously, the frontchamber 96 is being pressurized, at a slower rate, through therestriction 134, so the pressure acting on piston assembly 105 remainsunbalanced until after the seal 114 seals the axial port 99 in thepartition 95.

As shown in FIG. 12, the nozzle 70 has an off-set or jog bend 138 thatstops debris that may enter the nozzle 70 from entering into the frontchamber 96, and blocks transmission of radiant heat directly into thesignal generator 55 from the boiler interior. A U-shaped bend has alsobeen used successfully for this same purpose.

The process performed by the acoustic signal generator 55 to generate ahigh amplitude acoustic signal having a fast rise time includes pluggingthe front axial opening 113 in the front chamber 96 within the maincylinder 91 by positioning the plug 116 in the opening and pressurizingthe front chamber 96 and the rear cylinder 100 with gas. The rearcylinder 100 is then vented until the forwardly directed force exertedby the gas in the rear cylinder drops below the rearwardly directedforce exerted by the gas in the front chamber 96 on seal assembly 110,causing the piston assembly 105 and the seal assembly 110 to move to therear. Rearward movement of the intermediate seal 114 allows pressurizedgas to escape from the front chamber 96 to the rear side of thepartition 95 where it acts against the intermediate piston 107,accelerating the piston assembly 105 and attached seal assembly 110 tohigh speed prior to unplugging the front opening 113 in the frontchamber 96.

Accelerated to high speed, the piston 116 moves in less than 3milliseconds from a fully plugged position in the front axial opening orthroat 113 to a fully unplugged position, unplugging the opening 113 andreleasing pressurized gas suddenly to burst explosively from the cavitythrough the nozzle 70 into the boiler. Since the opening communicatesthrough the front closure 92 of the main cylinder 91 between the frontchamber 96 and the space inside the boiler, rapid removal of the plug116 allows sudden explosive release of the pressurized gas in the frontchamber 96 through the nozzle 70 into the boiler, producing a highamplitude acoustic signal 56 of about 185 db or greater having a suddensharp onset with a fast rise time. The signal 56 propagates sphericallyand is strong enough to reach across boilers as large as 95 feet acrossas a high amplitude signal, so it need not be aimed at particularreceivers. It can be detected by multiple receivers 60, enabling the useof advanced diagnostics such as furnace plane thermal tomography,illustrated in FIG. 24, wherein the average temperature on multiplescans across a plane at the furnace exit can be plotted to yieldinformation about the uniformity of the heat flux emanating from theboiler furnace. Flame impingement against the tube wall 32 can bedetected as shown in FIGS. 25 and 26 by arranging the signal generators55 and the receivers 60 to produce signal paths alongside the tube walls32. Thermal NO_(X) can be monitored as shown in FIG. 27 by arrangingseveral receivers 60 to take readings of gas temperatures as the gaspasses through the boiler to ensure that it does not have any extendedtime above 2700° F., above which thermal NO_(X) formation isaccelerated.

Since the venting of gas from the rear cylinder is at a controlled rate,a gas cushion remains in the cylinder 100 which prevents the rear piston108 from making contact with the rear end of the cylinder 100. Thesignal generator is reset for the next signal by operating the valve 135to pass gas under pressure to the rear cylinder 100 through therestriction 137 and to the front chamber 96 through the restriction 134.Gas pressure in the rear cylinder pushes the piston assembly 105 forwardinto gentle engagement with the intermediate partition 95 and the seal114 seals the axial port 99 in the intermediate partition 95, allowinggas pressure to build in the front chamber 96 to the designatedoperating pressure. The signal generator is now ready for operation toproduce the next acoustic signal.

The signal generator may be made in numerous ways, and it is intendedthat these various designs be encompassed by the claims appended hereto.The preferred embodiment of the acoustic signal generator is shown at140 in FIGS. 26-30. This acoustic signal generator 140 uses severalsimple and inexpensive machined parts of rugged design which fittogether simply and may be easily disassembled for routine maintenanceand repair.

Referring to FIG. 26, the sound generator 140 includes a straightcylinder 144 having a smooth cylindrical bore 146 and is externallythreaded at both a front end 148 and a rear end 152 to receive front andrear outer end nuts 154 and 156, respectively. The outer diameter of theinner ends of the outer end nuts 154 and 156 are stepped to a reduceddiameter, producing in external shoulder 157. Each outer end nut 154 and156 receives a hanger ring 158, shown in FIG. 26A. The hanger rings 158each have a pair of spaced hanger straps 160 by which the soundgenerator 140 is hung from a hanger beam 85 adjacent the boiler tubewall, as shown in FIG. 12. A pair of aligned holes 161 in the top end ofthe hanger straps 160 receive a shaft (not shown) on which the wheels 89are mounted for supporting the signal generator 140 from the hanger beam85, as shown in FIG. 12. Each hanger ring 158 has a stepped bore 162providing an internal shoulder 164 that engages the shoulder 157 on theexterior of the outer end nuts 154 and 156 to locate the axial positionof the hanger rings 158 relative to the outer end nuts 154 and 156. Asnap ring 166 fits into a groove 168 in each outer end nut 154 and 156to secure the hanger rings 158 in position on the outer end nuts 154 and156.

Each outer end nut 154 and 156 has an outer end flange 170 extendingradially inward for securing other components to the ends of thecylinder 144. At the rear end, the end nut 156 secures an annular portplate 173 to the end of the cylinder 144 by clamping an outwardlyextending flange 175 on the port plate 173 to the end of the cylinder144. The port plate has a cylindrical outer surface which fits with asnug sliding fit into the cylinder 144, and has an axial bore 177 whichis internally threaded. A series of axial holes 178 in the annular portplate 173 vents gas from the interior of the cylinder 144.

A rear cylinder 180 having external threads on its front end 182 and itsrear end 184 is threaded at its front end 182 into the internallythreaded bore 177 of the annular port plate. An inlet nut 190 isthreaded onto the rear end 184 of the rear cylinder 180 and traps an“O”-ring seal 188 to hermetically seal the inlet nut 190 to the rear endof the rear cylinder 180. A large diameter flexible hose 185, about 1½″in diameter and 36″ in length, is attached to the rear face of the inletnut 190, as shown in FIG. 12. The attachment hardware for connecting andhermetically sealing the hose 185 to the inlet nut is conventional andis not shown. A 2-way valve 135′ is attached to a hose plate 192 at theother end of the hose 185. In this embodiment, the 2-way valve 135′ ismore robust and durable than the 3-way valve 135 shown in FIG. 10. Ableed port 194 in the hose plate 192 continuously vents the hose 185 ata slow flow rate. The flow rate produced by high pressure gas supply 125is far greater than the flow rate through the bleed port 194 so the timeto pressurize the acoustic signal generator 140 is not significantlylonger than for the embodiment of FIGS. 18-21.

The hose 185 provides a reservoir volume that protects the valve 135′from pressure spikes and a source of pressurized air that pushes thecylinder 244 forward to close the seal 268. The reservoir volume is notcritical, but a reservoir volume that is too small does not provide thedesired diffusion of the pressure pulse, and a reservoir volume that istoo large does not reduce in pressure as fast a desired when the valve135′ operates and can reduce the gas cushion effect at the rear end ofthe rear cylinder 180 that safely decelerates the shaft/piston 240 afterreleasing the acoustic signal. The hose 185 should be between 24″ and54″ long, with the optimal length about 36″ for a 1½″ diameter hose.

The front outer end nut 154 has an inwardly extending flange 171 at itsfront end that clamps three components to the front end of the cylinder144: a cylinder liner can 200, a throat plate 205, and a nozzle weldring 210. The cylinder liner can 200 has a cylindrical body 212 thatfits with a snug sliding fit into the cylinder 144. The front end of thecylindrical body 212 ends in an outwardly extending radial flange 214and the rear end is formed as a partition 216 having a central conicalaxial port 218.

The throat plate 205 has a radial flange 222 by which the throat plateis clamped to the end of the cylinder 144 by the front outer end nut154. The outer cylindrical periphery of the throat plate 205 adjacentthe flange 222 has a groove which holds an “O”-ring seal 225 forhermetically sealing the throat plate to the cylinder liner can 200. Acentral axial throat 227 extends rearward from the throat plate 205 anddefines an axial bore 230 opening in the front end of the acousticsignal generator 140.

The nozzle weld ring 210 has a central axial opening 232 aligned withthe bore 230 in the throat plate 205. An annular lip 235 disposedcoaxially around the opening 232 provides a structure to which thebarrel 70 can be attached, as by welding or other suitable means ofattachment. The barrel 70 is preferably about 7′ long which provides acomfortable stand-off distance from the hot tube wall 32 and has theeffect of sharpening the acoustic pulse with little or no attenuation.

A piston assembly 240, shown installed in the acoustic signal generator140 in FIGS. 26, 29 and 30, includes an integral piston-shaft 242, shownin FIG. 28. The rear end of the piston-shaft 242 is machined as acylindrical rear piston 244 having a groove 246 for a piston ring 248and another groove 250 for a wear ring 252. The rear piston 244 tapersto a smaller diameter rear shaft 254 which then flares at anintermediate position to form an intermediate piston 260. A groove 262in the outer cylindrical surface of the intermediate piston 260 receivesa wear ring 264 which, with the wear ring 252 in the groove 250 of therear piston 244, supports the piston-shaft 242 for axial movement in thecylinder 144. An annular groove 266 in the front face of theintermediate piston 260 receives an “O”-ring seal 268 for sealing theinterface of the intermediate piston 260 and the tapering axial port 218in the partition 216 when the piston-shaft 242 is in its forward-mostposition illustrated in FIG. 26. The “O”-ring seal 268 is held in placeby a seal retainer ring 270 threaded onto a forward shaft portion 272 ofthe piston-shaft 242 which is threaded at 274 adjacent the intermediatepiston 260. A lock ring 276 is threaded onto the threads 274 against theseal retainer ring 270 and secured in place with a set screw 278.

As shown in FIG. 28A, the front end 280 of the forward shaft portion 272is threaded and threadedly receives a front piston barrel 282 which hasa threaded stud 284 extending forwardly therefrom. A piston donut 286slides onto the stud 284 followed by a washer 285 and is secured inplace by a nut 288 which is locked by a lock-wire or cotter pin. Thepiston donut 286 has one or two grooves 287 for receiving one orpreferably two piston rings 290.

Operation of the embodiment shown in FIGS. 26-30 is substantially thesame as the embodiment shown in FIGS. 18-21. One difference between thetwo embodiments is the gas supply circuit. In the first embodiment, thegas lines are external to the main cylinder 91; in the secondembodiment, the gas supply circuit is from the hose 185 through anopening 291 in the fitting 190 and into the rear cylinder 180. Athreaded plug (not shown) in an internally threaded end 292 of an axialbore 293 in the piston-shaft 242 has a small diameter hole (not shown)drilled therethrough to admit pressurized air into the bore 293. Thatpressurized air is conveyed through the bore 293 and into the frontchamber 96′ by which the front chamber is pressurized after the pressureacting against the rear face of the piston 244 moves the piston-shaft242 fully forward and seals the front chamber 96′ with the seal 268.

The signal processor 65, shown schematically in FIGS. 31-33, includes atime-of-departure module 145, a time-of-flight module 150, and atemperature calculation module 155. The time-of-departure module 145locates the beginning of the acoustic signal from the acoustic signalgenerator, using a level-trigger algorithm. The beginning of theacoustic signal from the acoustic signal generator is defined as thefirst time the signal amplitude exceeds a selected percentage (e.g.150%) of the maximum signal amplitude of the background noise receivedon the detector 57 for the acoustic signal generator. Instead of usingthe detector 57 and the tube 59 connecting the nozzle 70 to the detector57 to detect the acoustic signal, a pressure sensor or detector such asa piezoelectric element, or an optical, magnetic, capacitive or otherproximity sensor or detector could be mounted directly in the throat 90or the output nozzle 70 of the acoustic signal generator 55 to detectthe acoustic signal 56 or the movement of the piston 116 that releases ablast of air under pressure through the output nozzle to produce thedesired electrical signal indicative of the initiation of the acousticsignal 56 from the signal generator 55.

The time-of-flight module 150 analyses the signal received by thereceiver microphone 62 to facilitate differentiation between backgroundnoise and the acoustic signal so as to locate the beginning of theacoustic signal in the background noise. The time of flight module 150includes a digital prefilter for modifying the signal received in thereceiver to produce a modified signal having an increased ratio of theacoustic signal amplitude to the noise amplitude. The time-of-flightmodule also creates a stochastic model of the signal for determining thetime of onset of the acoustic signal in the receiver.

The digital pre-filter includes a linear prediction error filter orlinear whitening filter operating by an autocorrelation method thatmeasures N consecutive samples of the signal amplitude from the receivermicrophone, and predicts what the N+1^(th) signal will be from theprevious N samples. The predicted N+1^(th) signal is then subtractedfrom the actual measured signal value. In the preferred embodiment,twelve signal samples are used to make the prediction about thethirteenth, although other sample sizes could be used. This linearprediction error filter process is performed on each sample, resultingin a small amplitude modified or filtered signal having more of thecharacteristics of the acoustic signal from the signal generator 55. Theacoustic signal is more apparent in the modified or filtered signal. Toidentify the onset of the acoustic signal in the filtered signal, anaccurate and reliable method is to form a stochastic model of the signaland use it to find the most likely location of the onset of the acousticsignal in the filtered signal.

The stochastic model preferred in this embodiment of the invention is aMarkov model. It consists of two or more “states”. Each state behaveslike a stationary random variable that produces uncorrelated whiteGaussian noise. The model can move from state to state as timeprogresses.

A Markov model with three states is assumed for the signal. The firststate represents the background noise of the filtered signal without theacoustic signal imposed. The second state acts like the acoustic signal,and the third state models the filtered signal after the acoustic signalhas ended.

Since each state produces uncorrelated white Gaussian noise, the onlyunknown parameters are the mean value of the output and its variance.The filtered signal is normalized to zero-mean as part of thepre-filtering process, so only the variance must be estimated.

The variance of the first and third states are assumed to be the same,and are estimated using only signal samples known to contain onlybackground noise with the acoustic signal absent. Since the acousticsignal must arrive after it is generated by the acoustic signalgenerator, those samples that occur before the generation of theacoustic signal are used to estimate the variance in the first state.The variance of the samples from the second state is estimated fromsamples located directly around the sample with the sample with themaximum amplitude in the filtered signal.

The filtered signal from the receiver and the Markov model are usedtogether with the Viterbi algorithm, a well known algorithm that labelseach time index with a state. The transition between State One and StateTwo can be detected by determining the most probable time for the shiftfrom State One to State Two, and indicates the arrival of the acousticsignal.

To improve the reliability of the system, a number of checks are made toensure that the detected signal onset is physically reasonable andotherwise minimize the chances of indicating erroneous temperaturemeasurements. The system listens for a period of relative quiet insidethe boiler to take a measurement. The primary sources of noise insidethe boiler of high enough amplitude to interfere with the operation ofthe acoustic pyrometer are the soot blowers. When the time arrives forthe system to take a temperature measurement, the RMS value of thebackground noise as picked up by the receivers 60 is measured and nomeasurement is taken if the measured value exceeds a predeterminedthreshold, which can be selected for the particular boiler installationto produce the best combination of permissible measurement time andvalid measurements. Then the time of arrival of the signals detected bythe receivers 60 are compared to the time of generation of the signalsin the signal generator. If the arrival time of the acoustic signal isfound to lie in very close proximity to the beginning or end of thesampled interval, or if the amplitude of the signal is found to be smallcompared to the filtered background noise, the acoustic signal isdetermined to be invalid and is discarded. The temperature is evaluatedto determine if it is in a reasonable temperature range (e.g. 0-3500°F.) and is discarded if outside that range. The system keeps track ofthe last several measured temperatures and compares the latest measuredtemperature with those. If it is outside a reasonable range of likelychange (say, 300° F. in the normal measuring period of about twominutes) that measurement is presumed invalid and is discarded.

To produce a baseline temperature measurement at system start-up orafter the expiration of a validation period, e.g. 60 minutes, in whichno valid temperature measurements were taken, the system automaticallyperforms a “cold start” procedure to produce a baseline temperature forvalidation checking, that is, against which subsequent measurements canbe compared and discarded if they are outside the predeterminedvalidation range. The cold start procedure is to produce a number oftemperature measurements (e.g. 15) in rapid succession and average thosemeasurements. The number is large enough to dilute the effect anerroneous measurement, but small enough that the baseline can beproduced quickly. After the baseline temperature is established, it iscontinuously refined by discarding invalid measurements and comparingsubsequent measurements only to the lest several valid measurements.

The temperature module calculates the temperature of the open spacebetween the signal generator and the receiver. The path length throughthe open space is known, either by accurate measurement or bycalculation based on signal transit time at a known temperature. Thetransit time of the acoustic signal from the signal generator to thereceiver is a function of temperature, as expressed in the followingalgorithm, and the average gas temperature in the transit path throughthe open space across the boiler is readily calculated knowing thetransit time and the path length.$C_{s} = \sqrt{\frac{\gamma \quad R\quad T}{M}}$

Where:

γ=Ratio of specific heats

R=Universal Gas Constant

T=Temperature

M=Mean Molecular Weight of Gas

If path length=L, transit time=t, then $C_{s} = \frac{L}{t}$

In operation, the acoustic pyrometer measures the average gastemperature along a line 52 through the boiler or other open space ofknown dimension. An acoustic signal generator produces an acousticsignal with a high amplitude sudden onset. The signal is produced by asudden release of air under high pressure from a front opening in alarge cavity through a nozzle and into the open space. The processincludes plugging the front opening by positioning a piston in theopening. The gas in the cavity is pressurized and, when the acousticsignal is to be created, the piston is accelerated to high speed in theopening prior to unplugging the opening so that the piston moves at highspeed from a fully plugged position to a fully unplugged position andreleases the pressurized air explosively from the cavity to the externalspace. The acceleration of the piston uses the same air pressure in thecavity, so no external power source is needed.

A detector in the signal generator receives the signal from the signalgenerator and generates a first electrical signal which indicates thetime of generation of the acoustic signal by discharge of the signalgenerator. The first electrical signal is transmitted to the signalprocessor.

The acoustic signal propagates across a space of known distance in theboiler where it is received in a receiver which generates secondelectrical signals corresponding to amplitude and frequency of theacoustic signals in the receiver. The electrical signals from thereceiver are processed in a signal processor to produce a distinctdifferentiation between background noise and the onset of the acousticsignal in the receiver. The time of the onset of the acoustic signal inthe receiver is compared with the onset of the acoustic signal in thesignal generator to determine the transit time of the acoustic signal totraverse the space. The temperature of the gas in the space iscalculated based on the transit time of the acoustic signal across theopen space from the signal generator to the receiver, as describedabove.

The signal generators and receivers can be placed in the boiler innumerous configurations to achieve temperature data of interest to theboiler operator. For example, a single signal generator 55 and a singlereceiver 60 can be placed opposite to each other as shown in FIG. 7 toobtain the average gas temperature along the line between the signalgenerator 55 and the receiver 60. Information about the temperaturedistribution in a plane can be obtained by several receivers 60 in thatplane with a single generator 55 as shown in FIGS. 1, 2 and 9. Adetailed thermal map can be produced using multiple signal generators 55and receivers 60 arranged in the plane of interest, for example, in thefurnace exit plane as shown in FIG. 22 using known tomography techniquesto produce temperatures at points of intersection of the lines betweenthe signal generators 55 actuated serially in rapid succession and thereceivers 60.

Fireball centering can be achieved using two generators 55 and tworeceivers 60, as shown in FIG. 6. The signal generators 55 are actuatedseparately in rapid succession, and the signals received in thereceivers 60 are analyzed to detect non-uniform temperatures along thesides T1-T4. As illustrated, the equal temperatures along T1 and T3, andthe unequal temperatures along T2 and T4 indicate that the fireball 44is aligned equally between T1 and T3 walls, but is shifted away from theT2 wall toward the T4 wall. The orientation of the burners 42 can thenbe adjusted to shift the fireball toward the center of the firebox.Likewise, firewall impingement shown in FIGS. 23 and 24 can be detectedusing suitably placed signal generators 55 and receivers 60 (only onepair of which is shown).

In FIG. 25, vertical distribution of temperature above the furnace canbe obtained using a single signal generator and vertically spacedreceivers 60. This temperature distribution can give an indication ofdwell times above the critical temperature of 2700° F. at which thermalNO_(x) formation markedly increases. This temperature information givesthe boiler operator an early warning and an opportunity to adjust theburners in the furnace to reduce the temperature to a safe level.

By installing the signal generator 55 through the boiler nose tube wall69 as shown in FIGS. 34-36, a multitude of important gas temperaturescan be measured. The generator 55 is fitted with a long barrel 71 sothat the signal generator body can be placed outside the interior wall294 in the boiler nose, shown in FIG. 35, to isolate the signalgenerator from the hot interior chamber behind the boiler nose 69. Thereceivers 60 are placed in the side tube walls 32 and, as shown in FIG.36, at the ends of cable drop tubes 296 that extend through the top tubewall 298, through the “penthouse” space 300 and through the top wall 302of the boiler. This arrangement of the receivers 60 protects them fromexcessive temperatures inside the “penthouse” 300 and produces valuabletemperature data about the entrance plane into the pendant tube banks.

The bull nose 69 can provide useful access to receiver tubes 304, asshown in FIGS. 37 and 38, for receiving signals from signal generators55 mounted outside the boiler side tube walls 32 and conducting theacoustic signal through the tube 304 to a receiver 60 mounted on thecool side of a partition 306 across the bull nose 69, as shown in FIG.37, or a signal generator 55 in the bull nose 69, as shown in FIG. 36.

Obviously, numerous modifications and variations of the preferredembodiment described above are possible and will become apparent tothose skilled in the art in light of this specification. For example,many functions and advantages are described for the preferredembodiment, but in some uses of the invention, not all of thesefunctions and advantages would be needed. Therefore, we contemplate theuse of the invention using fewer than the complete set of notedfunctions and advantages. Moreover, several species and embodiments ofthe invention are disclosed herein, but not all are specificallyclaimed, although all are covered by generic claims. Nevertheless, it isour intention that each and every one of these species and embodiments,and the equivalents thereof, be encompassed and protected within thescope of the following claims, and no dedication to the public isintended by virtue of the lack of claims specific to any individualspecies. Accordingly, it is expressly intended that all theseembodiments, species, modifications and variations, and the equivalentsthereof, are to be considered within the spirit and scope of theinvention as defined in the following claims, wherein we claim:

What is claimed is:
 1. An acoustic signal generator for generating anacoutic signal with a high amplitude, sudden onset, comprising: a maincylinder having front and rear opposed ends and an axial opening in eachend; a partition in an intermediate portion of said cylinder dividingsaid cylinder into front and rear chambers, and an axial opening in saidpartition communicating between said chambers; a rear cylinder attachedto said rear cylinder end around said rear axial opening andcommunicating therethrough with said rear chamber; a piston assemblyhaving an intermediate piston in said rear chamber, and a rear piston insaid rear cylinder; a seal assembly connected to said piston assemblyand movable herewith, said seal assembly having a front plug and a frontseal coacting with said front plug to seal said front end axial opening,and an intermediate seal plugging said axial opening in said partition;said front seal is mounted on said front plug and is normally disposedon said front plug in a bore forming part of said front axial openingand is movable axially with said front plug; a pneumatic operatingsystem for charging said rear chamber of said main cylinder with gas ata first high pressure and for charging said rear cylinder with gas at asecond high pressure, said pneumatic operating system including acoupling for connection to a source of gas pressure and a remotelyoperated vent to allow said pressurized gas in said rear cylinder toescape, thereby reducing forwardly directed forces on said intermediatepiston in said rear chamber exerted by pressurize gas on said rearpiston, below rearwardly directed forces exerted by pressurize gas insaid forward chamber against said intermediate seal.
 2. An acousticsignal generator as defined in claim 1, wherein: said vent includes arestricted orifice through which gas is allowed to escape from said rearcylinder at a preselected slow rate; whereby a gas cushion remains insaid rear cylinder to decelerate said piston assembly and minimizedamage to said piston assembly.
 3. An acoustic signal generator asdefine in claim 1, wherein: said front seal is of smaller diameter thansaid intermediate seal.
 4. A method of generating an acoustic signalhaving a sharp, high amplitude onset, comprising: plugging an openinginto a cavity within a body by positioning a plug in said opening, saidopening communicating through said body between said cavity and externalspace outside said cavity; pressurizing gas in said cavity; acceleratingsaid plug to high speed prior to unplugging said opening; and unpluggingsaid opening by moving said valve at high speed from fully pluggedposition to a fully unplugged position and releasing said pressurizedgas suddenly from said cavity to said external space.
 5. A method asdefined in claim 4, wherein said valve accelerating step comprises:opening a port between said cavity; and exerting elevated gas pressuresuddenly against a large diameter piston connected to said valve.
 6. Aprocess of determining the arrival time of an acoustic signal propagatedthrough a noisy environment and detected in a receiver, comprising:digitally pre-filtering said acoustic signals received in said receiverto facilitate differentiation between background noise in said openspace and said acoustic signal so as to locate the beginning of saidacoustic signal in said background noise; said prefiltering includes, a)measuring N consecutive samples of said signal received in saidreceiver, b) predicting what an N+1^(th) signal will be from theprevious N samples, c) measuring said N+1^(th) sample to obtain anactual measured value of said N+1^(th) sample, d) subtracting saidpredicted N+1^(th) signal from said actual measured N+1^(th) signalvalue; and, e) repeating steps a)-d) with each new sample taken toproduce a small amplitude modified signal having more characteristics ofsaid acoustic signal from said signal generator; forming a stochasticmodel having two or more states, each state behaving like a stationaryrandom variable that produces uncorrelated white Gaussian noise, saidmodel able to move from state to state as time progresses, said modelhaving a first state representing said background noise of said filteredsignal without said acoustic signal imposed, and a second state actinglike said acoustic signal; normalizing said filtered signal to zero-meanas part of said pre-filtering process; estimating the statisticalvariance of the samples from said first state using signal samples knownto contain only background noise with said acoustic signal absent, usingsamples that occur before generation of said acoustic signal by saidacoustic signal generator; estimating the statistical variance ofsamples from said second state from samples located directly around saidsample with maximum amplitude in the filtered signal; and determiningthe most probable time for the shift from said first state to saidsecond state, and the most probable time for the arrival of saidacoustic signal, by labeling each time index with a state using saidfiltered signal from said receiver and said stochastic.
 7. A method ofcentering a fireball in a boiler furnace, comprising: separatelyactuating in rapid succession two signal generators placed in oppositesides of a fire box, and receiving signal produced by said signalgenerators in two receivers positioned opposite each other and on aplane transverse to a plane through both said signal generators;analyzing signals received in said receivers to detect non-uniformtemperatures along sides of said firebox; adjusting the orientation ofburners in said firebox to shift the fireball toward the center of thefirebox.